CROSS Safety Report
Specification issue with steel hollow sections
A reporter raises concerns after their structural engineer had failed to specify the steel grade used in the design for hollow sections on two projects.
Key Learning Outcomes
For architects and designers:
- There are different grades of steel for different purposes and it is important to make the appropriate selection and then to include that within the material specification
For civil and structural design engineers:
- As for architects and designers, the correct grade for purpose must be chosen
For the construction team:
Beware of substitutions offered that do not comply with the designer’s specification. In case of doubt, ask.
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The reporter is from a consulting firm. They found out that their structural engineer had failed to specify the sub-grade used in the design for hollow sections on two projects. A review was carried out to establish the impact of this on structural safety.
Project 1 – CHS sections used as props
CHS sections were used as props in temporary works. The utilisation of the CHS sections was 60% for hot formed steel members. The reporter’s firm had not however clearly communicated the requirements to use hot-formed sections in design documents or drawings. A check found that the sections were still safe if cold formed sections were used. This would have been very costly if revised utilisations would have resulted in overstressed members.
Project 2 – SHS sections used as supports
SHS sections were to be used in cable rack supports where the design was governed by deflections. The design report and drawings did not clarify the sub-grade to be used at the project start. Using cold formed sections rather than hot formed sections would have resulted in a very tricky situation with cost and programme impact if the design was not governed by deflection rather than stress.
The reporter says that if an engineer assumes hot formed steel and pushes their design close to unity but does not specify this on the drawings or in specifications clearly, the client may end up procuring cold rolled steel sections. These have less strength and inferior ductility properties so the affected structural elements could be overstressed. The lesson learned is to share this information and always state the sub-grade in the design report and in the drawings to provide clarity.
The lesson learned is to share this information and always state the sub-grade in the design report and in the drawings to provide clarity
Expert Panel Comments
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The reporter’s use of ‘sub-grade’ is possibly misleading as that term is normally used for the designation A, B, C etc which refer to products in the same strength grade but with varying values of toughness for selection when resistance to brittle fracture is a requirement. Refer to the Steel Construction Institution (SCI) Technical Report: Selection of steel sub-grade in accordance with the Eurocodes.
Sub-grade in steelwork usually refers to the toughness of the steel, and its ability to resist brittle fracture. A Charpy V-notch test is used to determine this. Thicker sections, and those subject to cold temperatures are more at risk, as are those with higher locked in weld stresses.
Hot v. cold rolled hollow sections
For SHS sections a further division is available as between hot rolled and cold rolled versions of the same serial sizes with hot rolled options generally having better buckling resistance. However, the main point is that designers must be precise when specifying products. One case is known of when a key connection failed because threaded bars of grade 4.6 were used instead of the grade 8.8 intended.
One case is known of when a key connection failed because threaded bars of grade 4.6 were used instead of the grade 8.8 intended
CROSS report 761 (Major UK steel manufacturer talks about steel substitution) from a UK steel manufacturer responding to a similar report (740 Common use of S235 cold rolled steel instead of S355 hot rolled steel) also notes that the cold rolled product does not require the same “quality, traceability and testing” as the hot rolled, and may require different welding processes and weld design considerations. Thus, the implications of substitution may go beyond simply the change in strength.
Specifying steel grade and sub-grade
It is essential that the correct grade and sub-grade of steel is specified for the appropriate use. Modern architecture often calls for externally exposed steelwork, for instance to provide support to brise-soleil. In such cases the external exposure of the steel demands an appropriate sub-grade to be specified to account for external exposure. There is much published information on this matter e.g. SCI 491 brittle fracture; selection of steel subgrade to BS EN 1993.1.10, by way of example.
An aspect to be considered is suppliers may offer inappropriate steel sub-grades because they have cheaper alternatives available.
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